This invention relates to a wireless communication system which enables a plurality of base stations to cooperate with one another in transmitting and receiving data to and from at least one wireless communication terminal.
In wireless communication, a wireless communication terminal at the cell edge cannot obtain a sufficient user rate because the signal-to-interference-and-noise ratio (SINR) is deteriorated by the attenuation with distance of a desired wave power from a base station to which the wireless communication terminal belongs and by the influence of interference waves from adjacent base stations.
As a technology that solves this problem and improves the user rate of a wireless communication terminal at the cell edge, base station cooperation technology is known in which base stations cooperate with one another in transmitting and receiving data to and from a wireless communication terminal.
The base station cooperation technology continues to be considered for Long Term Evolution (LTE) of 3rd Generation Partnership Project (3GPP) (see, for example, 3GPP TS 36.201 v8.1.0 (November 2007), and 3GPP TS 36.211, TS 36.212, TS 36.213 v8.4.0 (September 2008)), which has been decided all over the world to be employed as 3.9-generation wireless communication systems, and is expected to be incorporated in the standards of Long Term Evolution-Advanced (LTE-A) (see, for example, 3GPP TR36.814 V0.0.1 (September 2008)), which is the successor of LTE and one of the candidates for the fourth-generation wireless communication systems.
Known concrete data transmission methods in which base stations cooperate with one another include interference avoidance and network Multiple Input Multiple Output (MIMO) (see, for example, Laurence Mailaender, “Indoor Network MIMO Performance with Regularized Zero-Forcing Transmission,” IEEE ISSSTA 2008, pp. 129-132, August 2008).
Interference avoidance is a technology in which base stations each use Beam Forming (BF) to give high directivity to transmission signals in a manner that prevents signals of adjacent base stations from overlapping one another, thereby avoiding interference and improving SINR.
Network MIMO is an expansion of conventional MIMO transmission, which uses a plurality of antennas provided in one base station. Network MIMO uses a plurality of antennas provided in a plurality of base stations to perform MIMO transmission.
The following description of this invention focuses on network MIMO. It should be noted, however, that this invention is not limited to network MIMO and is applicable to other methods.
Network MIMO operation described here is classified into single-user transmission (SU transmission) and multi-user MIMO transmission (MU-MIMO transmission).
The premise of this invention is that wireless resources are multiplexed by Orthogonal Frequency Division Multiple Access (OFDMA), which is employed for downlink (downstream) in LTE. However, this invention is not limited to OFDMA and is applicable to other multiplexing methods such as Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA).
In SU transmission, a base station selects one wireless communication terminal and transmits data to the selected wireless communication terminal.
A wireless communication terminal in SU transmission receives pilot signals from a base station to which the wireless communication terminal belongs and from adjacent base stations, and estimates the channel.
Based on the result of estimating the channel, the wireless communication terminal calculates the quality of the channel to be obtained in the case where network MIMO transmission is used, the number of MIMO ranks, and a desired precoding matrix.
The wireless communication terminal transmits at least one of the above-mentioned calculated items and a list of base stations that participate in cooperative transmission to the base station to which the wireless communication terminal belongs, with the use of an uplink (upstream) control signal.
The base station that receives the control signal notifies information contained in the received control signal to a cooperation scheduler, which executes wireless resource allocation in base station cooperation.
The cooperation scheduler selects an optimum wireless communication terminal, a data transmission method, a subcarrier to be used, and the like based on the notified information, and notifies the selection results to the base stations that participate in cooperative transmission. An optimum wireless communication terminal can be selected for each subcarrier of OFDMA.
For example, network MIMO transmission between base stations 1 and 2 and wireless communication terminals 1 and 2 may be carried out such that the base stations 1 and 2 transmit by network MIMO transmission to the wireless communication terminal 1 on subcarriers 1 to 12, and to the wireless communication terminal 2 on subcarriers 13 to 24.
Network MIMO transmission methods that can be employed in SU transmission may include a method that uses Open-Loop MIMO transmission where a wireless communication terminal does not need to specify a precoding matrix, and the wireless communication terminal uses Minimum Mean Square Error (MMSE) or Maximum Likelihood Detection (MLD) as in normal MIMO, a method that uses Closed-Loop MIMO transmission such as Eigen Space Division Multiplexing (E-SDM), and a method that is used in, for example, transmit diversity such as Space Time Transmit Diversity (STTD).
In any of the methods given above, base stations participating in cooperative transmission exchange data necessary for cooperative transmission with one another before transmitting to a wireless communication terminal, generate signals in accordance with the employed method, and transmit the generated signals to the target wireless communication terminal. The wireless communication terminal decodes the signals in accordance with the method selected by the base stations, and obtains the data.
The wireless communication terminal that is the target of network MIMO transmission receives a desired signal from the base stations participating in cooperation, and the channel capacity is therefore markedly improved in any of the methods given above.
MU-MIMO transmission is an application of MIMO in which data is transmitted to a plurality of wireless communication terminals.
In MU-MIMO transmission, as in transmission to a single wireless communication terminal, a wireless communication terminal receives pilot signals from a base station to which the wireless communication terminal belongs and from adjacent base stations, and estimates the channel.
Based on the result of estimating the channel, the wireless communication terminal calculates the quality of the channel to be obtained in the case where network MIMO transmission is used, the number of MIMO ranks, a desired precoding matrix, and channel matrices of a plurality of base stations.
The wireless communication terminal transmits at least one of the above-mentioned calculated items and a list of base stations that participate in cooperative transmission to the base station to which the wireless communication terminal belongs, with the use of an uplink control signal.
The base station that receives the control signal notifies information contained in the received control signal to a cooperation scheduler, which executes wireless resource allocation in base station cooperation.
The cooperation scheduler selects an optimum combination of wireless communication terminals, a data transmission method, a subcarrier to be used, and the like based on the notified information, and notifies the selection results to the base stations that participate in cooperative transmission. An optimum combination of wireless communication terminals can be selected for each subcarrier of OFDMA.
For example, network MIMO transmission between base stations 1 and 2 and wireless communication terminals 1, 2 and 3 may be carried out such that the base stations 1 and 2 transmit by network MIMO transmission to the wireless communication terminals 1 and 2 on subcarriers 1 to 12, and to the wireless communication terminals 2 and 3 on subcarriers 13 to 24.
Network MIMO transmission methods that can be employed in MU-MIMO transmission may include a method that uses Zero Forcing (ZF) where the transmission side performs precoding with the use of an inverse matrix of a channel matrix, and a method that uses Dirty Paper Coding (DPC) where the channel capacity is improved by utilizing information about an interference signal.
ZF can be implemented based on a simple principle, but has a problem in that amplification exceeding the upper limit of transmission power is necessary depending on the inverse matrix of a channel matrix, which degrades the channel capacity.
DPC, on the other hand, is superior to ZF in terms of channel capacity but has a problem in that the amount of calculation is large. One of known DPC implementation methods that alleviate the problem is a method that uses LQ decomposition.
In the method that uses LQ decomposition, a channel matrix is decomposed into a lower triangular matrix and a product of unitary matrices. The transmission side executes advance equalization processing based on the lower triangular matrix, and executes precoding through Hermitian transposition of the unitary matrices. This procedure requires an amount of calculation that can be implemented in practice and, because unitary matrices are used in precoding, does not cause the extreme amplification of signal amplitude as in ZF. As a result, interference from adjacent cells is cancelled in a wireless communication terminal and the channel capacity is therefore improved.